Night Shift & Flux not working on MacBook Air 6,2 (early 2014) since macOS 10.13.2

Go to the 'Display' tab and check the 'Scaled' option instead of 'Default for display' (or vice-versa). Then, simply select whichever text option fits you better.

Once you select one, the display will reset automatically and you'll be having the correct display profile along with Night Shift working again.

Added the solution to Apple forums as well.

In my case, simply restarting my computer fixed this issue. Both F.lux and the Invert Screen option that I sometimes use at night (part of Accessibility features) were not working after upgrading to High Sierra. So, I suspected this issue was larger than just F.lux. I initially opened the above path but that led to an empty folder (there was no backgrounditems.btm file). As the suggestion involves both deleting backgrounditems.btm and restarting, I opted to see if maybe the latter would suffice and it did. In case it's helpful, I'm on a MacBook Pro (Retina, 15-inch, Late 2013).

UPDATE: F.lux and Invert Screen still work but now every time I connect or disconnect my laptop to an external LED Cinema Display or projector, the invert screen option gets triggered and must be manually un-inverted. Not a big deal but clearly a bug in High Sierra.